1. Field of the Invention
The present invention relates to a developing device using a two-ingredient type developer consisting of toner grains and carrier grains and an image forming apparatus and an image forming process unit including the same each.
2. Description of the Background Art
It is a common practice with an electrophotographic image forming apparatus to form a latent image on an image carrier, i.e., a photoconductive drum or belt in accordance with image data and develop the latent image with a developing device for thereby producing a corresponding toner image. The image forming apparatus uses either one of a one-ingredient type developer, i.e., toner and a two-ingredient type developer made up of toner and magnetic carrier grains. A developing device using the one-ingredient type developer is simple in construction and can be easily reduced in size. On the other hand, a developing device using the two-ingredient type developer is stable and long life and feasible for high-speed applications.
In the two-ingredient type developer, fine toner grains deposit on relatively large, magnetic carrier grains due to an electric force generated by friction acting between such two kinds of grains. When this type of developer approaches a latent image formed on the image carrier, an electric field formed between the image carrier and the latent image causes attraction tending to pull the toner grains toward the latent image to act. When the attraction overcomes adhesion acting between the toner grains and the carrier grains, the toner grains are deposited on the latent image for thereby developing it.
In the developing system using the two-ingredient type developer, the developer is repeatedly used while being replenished with fresh toner for making up for consumption. It is therefore necessary to maintain the toner content of the developer, i.e., the mixture ratio of the carrier grains and toner grains constant enough to insure stable image quality. To meet this requirement, a conventional developing device of the kind using the two-ingredient type developer needs a toner replenishing mechanism and a toner content sensor, resulting in a bulky construction and a sophisticated operation mechanism.
On the other hand, in the developing system using the one-ingredient type developer, toner grains deposit on the developer carrier due to an electric force derived from friction acting between the toner grains and the developer carrier or the magnetic force of a magnet disposed in the developer carrier. The toner grains deposit on the latent image formed on the image carrier because of the same mechanism as described in relation to the toner grains of the two-ingredient type developer. A developing device using the one-ingredient type developer can be reduced in size because it is not necessary to control the toner content of a developer. However, the number of toner grains available in a developing zone is too small to implement sufficient transfer of the toner grains to the image carrier. Therefore, the one-ingredient type developer is not feasible for a high-speed copier.
In light of the above, Japanese Patent Laid-Open Publication No. 9-22178, for example, proposes a developing device with automatic toner-content control capability that obviates the need for a toner replenishing device and a toner content sensor. Specifically, this developing device uses a developer carrier accommodating magnetic field forming means therein. In the developing device, a condition in which a two-ingredient type developer being conveyed by the developer carrier and fresh toner to be replenished contact each other is varied in accordance with the variation of the toner content of the developer. Consequently, the developer on the developer carrier is caused to automatically take in the fresh toner for maintaining a constant toner content. The developing device is therefore free from the drawbacks of the conventional developing device using a two-ingredient type developer, i.e., it is small size and simple in operation mechanism. It follows that the tow-ingredient type developer superior to the one-ingredient type developer in stability, service life and high-speed operation is desirable for such a developing device.
In the development system using a two-ingredient type developer, the closer the image carrier and developer carrier in the developing zone, the higher the resulting image density and the less the edge effect, as well known in the art. However, when the image carrier and developer carrier are positioned close to each other, it is likely that the trailing edge of a black solid image or that of a halftone solid image is lost. Further, it is likely that horizontal thin lines parallel to the axis of a sleeve or developer carrier become thinner than vertical fine lines or that solitary dots cannot be stably reproduced.
When the toner grains of the two-ingredient type developer are implemented as nonmagnetic grains, the toner grains of the developer deposited on the sleeve are scattered around due to a centrifugal force ascribable to the rotation of the sleeve. Such toner scattering becomes more conspicuous as the rotation speed of the sleeve is increased, so that nonmagnetic toner grains obstruct high-seed image formation.
To obviate toner scattering, the toner grains of the two-ingredient type developer may be implemented as magnetic toner grains. However, in the developing zone, magnetic toner grains are subjected to the magnetic force of the magnetic carrier grains directed away a photoconductive drum or image carrier. This magnetic force of the carrier grains, coupled with electrostatic attraction, makes it easier for the toner to leave the photoconductive drum, resulting in the omission of the trailing edge of an image.
Assume that the toner grains of the two-ingredient type developer are implemented as magnetic, spherical toner grains. Then, the toner grains have small surface energy each and easily move on the surfaces of the carrier grains. Therefore, the toner grains deposit on the surfaces of the carriers in an annular configuration at the position where the photoconductive drum and the tip of a magnet brush contact. Consequently, the bared carrier grain on the tip of the magnet brush faces the drum, aggravating the omission ascribable to toner drift, which will be described later specifically. There also occur the thinning of horizontal lines and unstable solitary dots due to the same mechanism, lowering image quality.
Today, toner produced by polymerization is attracting increasing attention and meets the demand for a small grain size capable of further enhancing image quality. Polymerization makes the individual toner grain more spherical and the grain size distribution narrower than conventional pulverization and therefore realizes high yield and cost reduction. In addition, polymerization consumes a minimum of energy on a production line.
To solve the various image defects stated above, Japanese Patent Laid-open Publication No. 2000-305360, for example, discloses a developing device provided with a particular flux density distribution in the normal to the surface of a sleeve. With the particular flux density distribution, it is possible to reduce the width of a developing zone in the direction of rotation of the sleeve, i.e., a nip width or to increase the density of a magnet brush in the developing zone. This prior art developing device will be described later more specifically.
There is an increasing demand for copied images or printed images with higher definition and higher resolution. The developing device taught in the above Laid-Open Publication No. 2000-305360 contributes to the enhancement of definition and resolution in that it improves the stability of solitary dots. However, even such a developing device is not satisfactory as to the reproducibility of a single dot whose resolution is as high as, e.g., 1,200 dpi (dots per inch) for the following reasons. First, because the width of the developing zone is reduced, the number of toner grains available in the developing zone is reduced, i.e., a sufficient amount of toner grains cannot be fed to the photoconductive drum. Second, when use is made of the two-ingredient type developer containing magnetic grains, the magnetic force acting on the toner grains interferes with the electrostatic force of a magnetic field tending to transfer the toner grains from the sleeve to the drum.
On the other hand, Japanese Patent Publication Nos. 6-82227 and 7-60273 each propose a particular developer having a small mean grain size and provided with a specific content of toner grains having grain sizes of 5 μm and below and a specific grain size distribution. Such a developer may enhance the definition and resolution of an image when applied to the developing system using a two-ingredient type developer. However, it was, in practice, difficult to reproduce a single dot with the ultrahigh resolution of 1,200 dpi by applying the above developer to the developing device disclosed in Laid-Open Publication No. 2000-305360 mentioned earlier.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 6-332237, 8-114986, 9-22178, 2000-39740, 321867, 2001-27849 and 2001-296743.